INTERLAMELLAR ADSORPTION OF ALCOHOLS .4. ADSORPTION PROPERTIES OF CRYSTALLINE SILICAS

Crystalline silicas (crystalline silicic acids) are intracrystalline-r eactive layered materials. Two types were studied: H4Si20O42 . xH2O an d HSi14O30 . xH2O. They show a pronounced preference of methanol and e thanol from mixtures with water, n-butanol, and toluene. The surface e xcess isotherms are U-shaped with long linear sections extending from x1 = 0.2 or 0.3 to x1 = 1 (x1 molar fraction of methanol or ethanol). Thus, at x1 greater-than-or-equal-to 0.2 or 0.3, only methanol or etha nol is adsorbed between the layers. The preference of H4Si20O42 for me thanol and ethanol from mixtures with water is as high as in silicalit e and NaZSM-5. With longer alcohols (propanol ... n-octanol), the isot herms become S-shaped, and molecules of component 2 (water, n-butanol, toluene) are also adsorbed in the interlayer spaces. Adsorption from mixtures of n-pentanol (or n-hexanol) and n-butanol produces unusual s urface excess isotherms. The cause is formation of paraffin-type bilay ers of pentanol or hexanol which, at a certain molar fraction of butan ol, collapse to monolayers of flat-lying alcohol molecules. When the a lcohol molecules lie flat in the interlayer space (basal spacing less than 2.3 nm), the volume of the adsorption phase, V(s), comprises the interlamellar volume V(int) (determined by X-ray powder techniques) an d the volume V(ex) of a few layers of alcohol molecules adsorbed at th e external surface. The ratio V(s)/V(int) varies between 1.2 and 1.5. When the layer separation becomes larger (paraffin-type bilayers of al cohols), V(int) exceeds V(s). This behaviour is also observed for hydr ophobized clay minerals in the presence of polar/unpolar liquid mixtur es.